The present invention relates to a decoding apparatus and a presumed motion vector calculating method for digital video signals which are utilized in television telephones, tele-conferences and so on.
In recent years, with the rapid development of television telephones and tele-conference apparatuses, a demand has arisen for a method of preventing degradation of the picture quality caused by transmission errors, particularly in a digital video signal decoding apparatus.
Generally, a digital video signal coding apparatus first performs motion compensation for each of the pixel blocks (for example, 16.times.16 pixels) which constitute the previous frame in order to remove correlation in time, and calculates a difference value of each pixel block between the previous frame and the current frame. In this case, the amount of information relating to the difference values is much smaller than the amount of information relating to the current frame. Then, the difference values and motion vectors used for the motion compensation are coded, and binary codes are outputted.
On the other hand, a digital video signal decoding apparatus decodes the binary codes outputted from the coding apparatus to obtain a difference value of each pixel block between the motion-compensated previous frame and the current frame, and a motion vector used for the motion compensation for that pixel block. The decoding apparatus is provided with a frame memory for storing the previous frame, and pixel blocks, which constitute the previous frame stored in the frame memory, are motion-compensated by using corresponding motion vectors. The difference values are added to the motion compensated pixel blocks in the previous frame to thereby reproduce the current frame. The reproduced current frame is stored in the frame memory. In this manner, the decoding apparatus allows the current frame to be reproduced only by the use of the difference values of the respective pixel blocks between the previous frame and the current frame and the motion vectors.
A conventional digital video signal decoding apparatus will be explained below with reference to FIG. 1.
As shown in FIG. 1, a conventional digital video signal decoding apparatus is composed of an input terminal 101, a decoder 102, a motion compensation circuit 103, a frame memory 104, an adder 105, and an output terminal 106.
Next, the relation between the respective constituent elements and the operation of the apparatus shown in FIG. 1 will be explained.
When a binary code is first supplied from a coding apparatus to the input terminal 101, the decoder 102 decodes a difference value of each pixel block between the previous frame and the current frame and a motion vector which was used for motion compensation of the pixel block in the previous frame. Then, the motion compensation circuit 103 uses the motion vectors inputted from the decoder 102 to motion-compensate the respective pixel blocks in the previous frame read out from the frame memory 104. The adder 105 adds the difference values of the respective pixel blocks between the previous frame and the current frame outputted from the decoder 102 to the values of the pixel blocks in the motion-compensated previous frame outputted from the motion compensation circuit 103 to reproduce the current frame. The reproduced current frame is delivered from the output terminal 106 and stored in the frame memory 104 for use in the decoding of a next frame.
Thus, the conventional decoding apparatus described above, as an example, can decode a video signal from a small amount of information including the difference values of the respective pixel blocks between the previous frame and the current frame and the motion vectors, and hence efficient video signal transmission can be accomplished.
However, in the above-described conventional structure, if a part of a bit stream is lost on a transmission path for some reason, a pixel block may appear in which the difference value between the previous frame and the current frame and the motion vector cannot be decoded, whereby pixels in such a pixel block in the previous frame which have not been subjected to motion compensation are displayed in a portion of the frame, causing a problem that the picture quality is greatly degraded, particularly in a moving portion.